The present invention relates to a process for the fractionation and purification of aromatic polyamine mixtures and the use thereof.
The preparation of aromatic polyamines and polyamine mixtures, particularly of the diphenylmethane series, is described in numerous patent applications and patents, as is the use of said products. Outstanding importance is attached to the use of said products as raw materials for the preparation of isocyanates, usually by reaction of the polyamine mixtures with phosgene according to the generally conventional and well known methods.
In many cases, however, the resulting isocyanates and isocyanate mixtures are not obtained in the form and composition subsequently used in preference in the isocyanate stage but have to be converted beforehand to the appropriate form for use by means of, in some case, time-consuming work-up and separation processes. Suitable polyamine preliminary stages which may be converted in a less time-consuming manner to the isocyanate use forms are in many cases difficult to produce or unobtainable in terms of process technology, or economically unattractive.
An example is the production of 4,4'-diisocyanato-diphenylmethane which is important for the preparation of valuable polyurethane materials, the amine preliminary stage of which may be obtained usually from aniline and formaldehyde only together with isomers, particularly the 2,4'-isomers and polyamines with a relatively large number of functional groups. Although said constituents are the basis for the likewise sought after isocyanates, the separation of the crude isocyanates into the isocyanates and isocyanate mixtures suitable for subsequent use is not easy. Initially, a part of the binuclear compounds are usually separated from the remainder. The 4,4'-diisocyanato-diphenylmethane from the binuclear fraction is then separated from the other isomers in a second distillation stage requiring many separation stages.
Even in recent times, the 2,4'-isomer in the enriched form has gained increasing importance as a polyurethane raw material, but may be enriched compared with the 4,4'-isomer and separated from the 2,2-isomer possibly present only by an elaborate distillation procedure.
Isomer separation methods or enrichment methods within the fraction of polynuclear-homologues, or of polyfunctional amine constituents, and of isocyanates of the diphenylmethane series, are practically unknown.
4,4'-Diaminodiphenylmethane is also attracting increasing interest as a raw material for di-(4-isocyanato-cyclohexyl)methane, the form of 4,4'-diisocyanato-diphenylmethane hydrogenated on the nucleus, the provision of suitable aromatic polyamine mixtures for the hydrogenation stage with the highest possible content of 4,4'-diaminodiphenylmethane with simultaneously the smallest possible proportion of 2,4'-diaminodiphenylmethane being very expensive.
It is well-known that amines may be separated in certain cases by partial conversion to their salts, wherein the different base strengths inter alia may be used. These are usually monoamines with very different base strengths. Such disproportionation effects in two-phase systems have also been described already for aromatic polyamine mixtures, particularly of the diphenylmethane series (German Auslegeschriften 2,238,319 and 2,528,697).
As a result of the numerous components present in such a mixture, the amino groups of which hardly differ at all in terms of type practically all of them are arylamino groups--the effects are not particularly great and pronounced to be of interest for direct use with simple means.
The object of the present invention was to provide a process which allows aromatic polyamine mixtures to be fractionated and purified in a simple manner so that isomers are obtained in a pure or enriched form.